Cooperative Intelligent Transport Systems (C-ITS) refer to the set of applications and use cases based on the exchange of messages between traffic participants in order to improve road safety, traffic efficiency or travel comfort. In this context, messages can be exchanged between vehicles (Vehicle-to-Vehicle communication, V2V communication), between vehicles and consumer electronics devices such as smartphones or wearables (Vehicle-to-Pedestrian communication, V2P communication), and between vehicles and road infrastructure such as traffic lights or road signs (Vehicle-to-Infrastructure communication, V2I communication). This is overall referred to as vehicle-to-everything communication or V2X communication. In the case of cooperative aware (CA) traffic safety applications, each vehicle broadcasts regularly beacons with its position, velocity, trajectory and other useful data to the vehicles in its vicinity.
The European and American systems for C-ITS are known as ETSI ITS-G5 and Wireless Access in Vehicular Environments (WAVE). Both the American and the European systems for C-ITS operate at 5.9 GHz and are based on the IEEE 802.11p standard, which defines the physical (PHY) and Medium Access Control (MAC) layers based on previous standards for Wireless Local Access Networks (WLAN). In particular, the IEEE 802.11p standard incorporates, with some modifications, the physical layer based on Orthogonal Frequency Division Multiplexing (OFDM) from the IEEE 802.11a standard, and the MAC layer based on the Enhanced Distributed Channel Access (EDCA) from the IEEE 802.11e standard. The EDCA protocol is contention-based and uses Carrier Sense Multiple Access (CSMA) with Collision Avoidance (CSMA/CA) and four different access classes (ACs). In addition to the EDCA protocol, several solutions such as De-centralized Control Congestion (DCC) or Self-organizing Time Division Multiple Access (STDMA) have been proposed in the literature and the standardization groups to improve the performance of the channel access in the IEEE 802.11p standard.
The EDCA protocol included in the IEEE 802.11p standard for channel access is not well suited to ensure the QoS requirements of C-ITS in congested scenarios. According to the CSMA/CA mechanism on which the EDCA protocol is based, a 802.11p mobile station, which could be implement within a vehicle, first senses the channel during a listening period before the transmission of information. If the channel is not occupied, the mobile station transmits immediately. Otherwise, the mobile station performs a back off procedure. The back off time that a mobile station must wait before transmission is random but limited by a parameter known as the contention window (CW). A collision occurs when multiple mobile stations try to transmit at the same time. The EDCA protocol includes four priority levels known as ACs with different listening periods and CWs. ACs with higher priority generally allow faster channel access by using shorter listening periods and shorter CWs.
In congested scenarios, i.e. when a high number of 802.11p mobile stations are competing to access the channel, the transmission of information can be delayed beyond acceptable values for C ITS due to the high probability of collision in the EDCA protocol. The DCC mechanism has been proposed as a manner to reduce the channel congestion with the EDCA protocol. When DCC is used, the transmission parameters (including power, periodicity of broadcasting beacons and carrier sense range) are updated according to the channel load observed by each 802.11p mobile station. Typically the response is to reduce the transmit power or the broadcasting periodicity when a higher channel load is detected.
Another proposed mechanism to improve the channel access in 802.11p mobile stations is to replace or adapt the CSMA/CA mechanism of the EDCA protocol with distributed time division multiplexing access (TDMA) mechanisms such as Self-Organized TDMA (STDMA). In this case, all mobile stations are assumed to be synchronized in the time domain, which is divided into frames, which in turn are divided into slots. The duration of each slot typically corresponds to the transmission time of one broadcasting beacon. Each mobile station selects randomly among free slots, a number of slots within each frame to transmit. In case all the slots are occupied, the mobile stations select the slot that is occupied by the mobile station located furthest away to minimize the interference. In order for STDMA schemes to operate correctly, it is important that all mobile stations are synchronized in the time domain, which it is generally achieved by means of a Global Navigation Satellite System (GNSS) such as the Global Positioning Service (GPS).
802.11p mobile stations are designed to operate in a distributed manner without any kind of centralized management. This is usually referred to as Vehicular Ad-hoc Networks (VANETs). This means that each IEEE 802.11p mobile station has to configure its parameters for transmission and channel access independently. In this context, the CSMA/CA mechanism results in unbounded delays for the messages as a result of the random back-off procedure and the high probability of channel collisions when the number of mobile stations competing for channel access increases. Distributed TDMA schemes like STDMA, on the other hand, are capable to provide deterministic channel access probabilities for all transmitting mobile stations. Nevertheless, they still rely on distributed resource allocation, and therefore, are prone to interference situations between transmitting mobile stations, which in turn, can delay the successful reception of messages beyond tolerable delays. Once a time slot selection has been made in STDMA, it continues to be used for 3 to 8 seconds, so that the relative position between mobile stations might change up to a few hundred meters during this time. This is especially critical in situations where mobile stations are travelling with very high velocity relative to each other, such as in the case of vehicles travelling in different directions on a motorway. In this context, the resource allocation performed by STDMA cannot react on time to the fast variations of the VANET, and therefore, the exchange of messages between mobile stations is prone to errors as a result of severe interference.